A sensorless control device of a permanent magnet synchronous motor needs to estimate the magnetic pole direction because the position of the magnetic pole at the time of starting a rotor from a stop state of the rotor is estimated using the magnetic saliency of the rotor. Methods that employ magnetic saturation to estimate the magnetic pole direction have been proposed.
In the estimation methods of the related art, the magnetic pole direction is estimated based on the assumption that the inductance in the magnetic flux direction of a magnet decreases monotonically with respect to an increase in the current in the N-pole direction. Consequently, this technology cannot be applied if the inductance in the magnetic flux direction of the magnet has a maximum point and does not decrease monotonically with respect to an increase in the current in the N-pole direction.